1. Field of the Invention
An apparatus consistent with the present invention relates to a scrolling unit and a projection system which forms a color image using the scrolling unit and, more particularly, to an endless track-like color scrolling unit which is installed on a single light path to handle all plural color beams and which can perform color scrolling upon rotation, and a projection system using the color scrolling unit.
2. Description of the Related Art
Projection systems are classified into either 3-panel projection systems or single-panel projection systems according to the number of light valves that are used. The light valves control the on/off operation of light emitted from a high-output lamp, used as a light source, on a pixel-by-pixel basis and thus, forming a picture. Single-panel projection systems can have a smaller optical system than the three-panel projection systems. However, these single-panel projection systems provide only ⅓ of the optical efficiency of the three-panel projection systems because red (R), green (G), and blue (B) colors into which white light is separated are used sequentially. Hence, attempts to increase the optical efficiency of single-panel projection systems have been made.
Generally, in a single-panel projection system, light radiated from a white light source is separated into R, G, and B color beams using color filters, and the three color beams are sequentially transmitted to a light valve. The light valve operates according to the sequence of color beams received and creates images. As described above, a single-panel projection system uses color beams sequentially, therefore, the light efficiency is reduced to ⅓ of the light efficiency of a three-panel projection system. According to one color scrolling method designed to increase the optical efficiency of a single-panel projection system, white light is separated into R, G, and B color beams, and the three color beams are simultaneously sent to different locations on a light valve. Since an image cannot be produced until all of the R, G, and B color beams for each pixel reach the light valve, the color beams are moved at a constant speed using the color scrolling method.
In a single-panel scrolling projection system, as shown in FIG. 1, white light emitted from a light source 100 passes through first and second lens arrays 102 and 104, and a polarization conversion system (PCS) 105, and is separated into R, G, and B color beams by first through fourth dichroic filters 109, 112, 139, and 122. More specifically, the red beam R and the green beam G, for example, are transmitted by the first dichroic filter 109 and advance along a first light path L1, while the blue beam B is reflected by the first dichroic filter 109 and travels along a second light path L2. The red beam R and the green beam G on the first light path L1 are separated by the second dichroic filter 112. The second dichroic filter 112 transmits the red beam R along the first light path L1 and reflects the green beam G along a third light path L3.
First through third prisms 114, 135 and 142 are disposed in the first through third light paths L1, L2, and L3, respectively. The light emitted from the light source 100 is separated into the R, B, and G beams, and they are then scrolled while passing through corresponding first through third prisms 114, 135, and 142. The first through third prisms 114, 135, and 142 rotate at a uniform speed such that R, B, and G color bars are scrolled. The B and G beams that travel along the second and third light paths L2 and L3, respectively, are transmitted and reflected by the third dichroic filter 139, respectively, and then combined. Finally, the R, G, and B beams are combined by the fourth dichroic filter 122. The combined beam is transmitted to a light valve 130 via a polarization beam splitter (PBS) 127. The light valve 130 forms a picture.
A condensing lens 107 is disposed next to the PCS 105, and light path correction lenses 110, 117, 131, 137, and 145 are disposed along the first through third light paths L1, L2, and L3. Condensing lenses 120 and 140 are disposed between the second and fourth dichroic filters 112 and 122 and between the third and fourth dichroic filters 139 and 122, respectively. A focusing lens 124 and a polarizer 125 are disposed in the light path between the fourth dichroic filter 122 and the PBS 127. Light path changers, for example, mirrors 118 and 133, are disposed in the first and second light paths L1 and L2, respectively.
The periodic scrolling of the R, B, and G color bars due to rotation of the first through third prisms 114, 135, and 142 is illustrated in FIG. 2. Scrolling represents the movement of color bars formed on the surface of the light valve 130 when the first, second, and third prisms 114, 135, and 142 corresponding to R, B, and G colors are synchronously rotated. As described above, when R, G, and B color bars circulate one time, one frame of a color image is formed.
A color image obtained by turning on or off the individual pixels of the light valve 130 on or off according to an image signal is magnified by a projection lens (not shown) and projected onto a screen.
First, second, and third slits 113, 134, and 143 are disposed in front of the first, second, and third prisms 114, 135, and 142, respectively, and control the divergence angle of incident light. The widths of the color bars vary according to the widths of the first, second, and third slits 113, 134, and 143. If the slit widths decrease, the R, G, and B color bars are narrowed such that black bars K are formed between adjacent color bars as illustrated in FIG. 3A. On the other hand, if the slit widths increase, the R, G, and B color bars are enlarged such that overlapping portions P are formed between adjacent color bars as illustrated in FIG. 3B.
Since the conventional projection system uses different light paths for each color as described above, a light path correction lens must be included for each of the colors, components for unifying the separated light beams must be further included, and separate components must be included for each color. Hence, the conventional optical system is bulky, and the manufacturing and assembly thereof is complicated, thus degrading the yield. In addition, three motors (not shown) for rotating the first, second, and third prisms 114, 135, and 142 generate a lot of noise during operation. Thus, the projection system adopting three motors is manufactured at a greater cost than a color wheel type projection system which utilizes a single motor.
In order to produce a color picture using a scrolling technique, color bars as shown in FIG. 2 must be moved at a constant speed. Hence, the conventional projection system must synchronize the light valve 130 with the three prisms 114, 135, and 142 in order to achieve proper scrolling. However, controlling the synchronization is not easy. Further, due to the individual circular motions of the first, second, and third prisms 114, 135, and 142, the color scrolling speed by the three scrolling prisms is irregular, consequently deteriorating the quality of the resultant image.